In times of UK national peril, the spirit of King Arthur is never far away, with his promise of justice, chivalry and a better England for all. So it is with this new film for older kids that lands in the midst of Brexit chaos to tell a tale of young heroes banding together under the leadership of a modern-day Arthur - pre-teen Alex Elliot (Louis Ashbourne Serkis).

A high school outsider targeted by bullies, Alex nonetheless has a big enough heart to be the Once and Future King, and proves it by plucking the ancient sword Excalibur from the stone when he stumbles upon it sticking out of a block of concrete on a London building site.

Serkis, by the way, is the son of Andy 'Gollum' Serkis, the actor, director and Lord of the Rings icon, and looks a lot like him, so the film is flashing its heroic fantasy credentials from the opening frame. He's a likeable young star, but royally upstaged with the arrival of Angus Imrie as the sorcerer Merlin.

Taking the form of a teenager (the adult Merlin is played by Patrick Stewart in a couple of brief scenes), Merlin waltzes into school looking and sounding like a Harry Potter nerd with ambitions to join the Royal Shakespeare Company. He's comic gold.

Rhianna Dorris, Louis Ashbourne Serkis, Angus Imrie, Dean Chaumoo and Tom Taylor in a scene from The Kid Who Would Be King. Kerry Brown

Meanwhile, deep in the bowels of the earth, the reptilian sorceress Morgana, played by Mission: Impossible's breakout star Rebecca Ferguson, is returning to life and planning the enslavement of Great Britain with the aid of her army of undead knights and her ability to command tree roots like so many scary tentacles.

Rebecca Ferguson as the evil Morgana.

Writer-director Joe Cornish told a similar story with his 2011 debut Attack the Block, in which South London drug dealers and teen hooligans joined forces to combat an unexpected alien invasion.

This new film is firmly aimed at the YA literature set and manages to surprise despite hewing closely to Arthurian legend, not to mention other movies about the Knights of the Round Table. In fact, some visual touches featuring the Lady of the Lake are direct steals from John Boorman's 1981 epic Excalibur - in which Patrick Stewart also appeared - but at least they're stealing from the best.

Patrick Stewart and Louis Ashbourne Serkis in The Kid Who Would Be King. Kerry Brown

Crucially, the plot turns on the moral values that underpin the Camelot myth, and how desperately they're needed in today's world. But that doesn't mean the movie short-changes on action, with a rousing climax that turns the school into a castle besieged by zombie knights on horseback, up against an army of bolshie British schoolkids. The smart money is on the kids.